Metal container



Patented Aug. 22, 1933 UNITED STATES PATENT OFFICE METAL CONTAINER N0 Drawing. Application September 2, 1930 Serial No. 479,413

3 Claims.

My invention relates to the production of pressure vessels and especially to means of constructing such vessels so as to prevent caustic embrittlement thereof.

Caustic embrittlement is a term commonly referred to in connection with heated pressure vessels, such as riveted steam boilers but the embrittlement may occur in any container Where the metal of which the vessel is composed, is subjected to high fibre stress and is simultaneously exposed to alkali hydroxides or to alkali salts which break down and produce alkali hydroxides. The ultimate eifect of caustic embrittlement is the production of cracks at points of high localized 0 stresses, such as at and near the seams of the vessels. These cracks extend through the body of the metal, and greatly reduce its strength. The characteristics of these embrittlement cracks may be summarized as follows: They start on the dry side of the plate, and run in irregular directions, in general from one point of high stress, such as a rivet hole, to another, without following the line of what is generally considered the line of maximum stress. A microscopic examination of the cracked plate reveals the fact that the crack progresses around the grain boundaries, or is intercrystalline. (Embrittlement of Boiler Plate, by S. W. Parr and F. G. Straub, Bulletin No. 177, June 1928, Univ. of Illinois Engineering Experiment Station; especially pages 11 and 12.)

In steam boilers the failures which are due to embrittlement develop most rapidly where a concentration of alkali occurs and where unusually large localized stresses occur, for example at the margins of rivet roles. The usually accepted explanation of the embrittlement of steam boilers is that alkali hydroxides or alkali salts of weak acids, such as carbonates are present in the boiler feed water or are introduced into the boilers-by adding water softening compounds to the feed water or by introducing cleaning compounds in the boilers. The alkalicompounds slowly work their way into the tightest seams where they become concentrated through evaporation and gradually break down and form alkali hydroxides.

If frequent costly tests are not made to detect weaknesses due to this cause in steamboilers and other containers, failure of the containers or explosions of the boilers accompanied with serious consequences are a common result.

I have found that the failures due to caustic embrittlement of vessels are practically avoided and safer and more permanent structures are provided by constructing the vessels of certain iron-chromium alloys containing about 5% to 30% chromium in which the metal is predominantly in the alpha chrome ferrite condition. By the latter term I mean that in these alloys the amount and distribution of alpha ferrite which contains the chromium in solid solution is sufiicient to render the structure free from a continuous austenitic structure.

As an illustration of one embodiment of my invention, a steam boiler is made by taking rolled plates of iron-chromium alloy of the following analysis:

Per cent Chromium 19.4 Manganese -4 0.40 Silicon 0.9 Carbon; 0.10

and the remainder iron with small amounts of other impurities. Rivet holes are provided in appropriate places and the plates are riveted together in the manner well known in the art. The alloy of which this vessel is composed consists largely of metal in the alpha chrome ferrite condition. The relative resistance to embrittlement of this boiler as compared to boilers composed of carbon steel is shown in a table hereinafter to be explained.

The greatest resistance to embrittlement is present when the metal is in the alpha chrome ferrite condition but austenite may be present in considerable amounts if the austenite is disseminated sothat the austenitic structure is not continuous, that is to say, the aggregates of this compound are separated so that they do not form chains of the compound through the structure.

Thebest results have been obtained with alloys containing at least 12% chromium but lower chromium alloys within the range referred to canbe used. Alloys containing more than about 20% chromium are costly to manufacture and for economic reasons the alloys containing about 12% to 20% chromium are preferred.

Iron chromium alloys usually contain small amounts of elements such as carbon, manganese, silicon, sulphur, and phosphorus which have practically no eifect on the resistance of the alloys to embrittlement. Manganese is a desirable addition to-give the alloy strength and ductility. It promotes the formation of austenite but it may be present in quite large amounts, for example about 8.0%. Additions of nickel and other elements in amounts that produce alloy structures which are predominantly austenitic, destroy the resistance of the alloy to caustic embrittlement, but nickel may be present in small amounts which do not produce a continuous austenitic structure.

Theiron generally amounts to about (0% but it may be reduced to as low as 60% in alloys which contain manganese and about 16% or more of chromium. In such alloys, be increased to about 10% or 15%. For example, an alloy having high resistance to caustic embrittlement and having an alpha chrome ferrite structure may contain approximately 22% chromium, 10% manganese and 68% iron.

The following table shows a series of alloys which were tested under conditions which produce the same caustic embrittlement as that which has been referred to, but the conditions used in the tests produce the embrittlement more rapidly than it is usually produced in practice. The tests show comparatively the resistance to embrittlement of the parts of structures which are stressed and subjected to solutions of alkali hydroxide. The tests were conducted'by immersing test pieces composed of the alloys in a solution containing 300 grams per liter of sodium hydroxide which were contained in a closed bomb, and by stressing the test pieces with loads which were about 10% above their yield points. The contents of the bomb were heated so that a steam pressure of about 500 pounds gauge was maintained.

%0 %Or %Mn %Si %8 %P %Ni A 0. 18 None 0. 30 0.02 Low Low None 0. 03 2. 86 0.06 0. 06 0. 032 0. 002 None 0 05 18. 87 51 31 Low Low 8. l6 07 16. 83 0. 34 0. 51 0. 012 0. 004 None l0 19. 43 0. 40 0. 92 0.002 0. 003 None 08 12. 51 0. 33 0. 27 Low LOW 0. l5

Specimen A was a sample of standard boiler plate and it contained no chromium. In specimen B the content of chromium was small. Specimen C was predominantly au'stenitic and contained a piece A failed within 24 hours from the emthe manganese may large amount of chromium. Test brittling action of the caustic soda and test pieces B and C failed in less than one hour. Specimens D, E and F which contained chromium within the-above described range and in whichthe metal was mostly in the alpha chrome ferrite condition had not failed after several months. Specimens containing higher and lower amounts of chromium than test pieces E and F with the above described alloy structure gave results similarto specimens D, E, and F.

Although my invention has been described more specifically with reference to the construction of a riveted steam boiler, it is to be understood that other vessels which are subjected to caustic alkalis and to comparatively high fibre stresses are within the scope of my invention, as for example the so-called seamless cast structures or welded structures.

I claim:

1. A vessel designed for use under conditions adapted to produce caustic embrittlement in carbon steel, parts of said vessel exposed to such conditions being composed of a ferrous alloy containing 5% to 30% in its metallographicstructure, predominantly of alpha chrome ferrite.

2. A vessel designed for use under conditions adapted to produce caustic embrittlement in carbon steel, parts of said vessel exposed to such conditions being composed of a ferrous alloy containing 12% to 20% of chromium, and consisting, in its metallographic structure, predominant- 1y of alpha chrome ferrite.

3. A vessel designed for use under conditions adapted to produce caustic embrittlement in carbon steel, parts of said vessel exposed tosuch conditions being composed of a ferrous alloy containing 5% to 30% of chromium, and about of chromium, and consistin 8% of manganese, and consisting, in its metalalpha AUGUSTUS B. KINZEL. 

